Spatial and Temporal Variability of Serpentinization in the Izu-Bonin-Mariana Convergent Margin

The degree of serpentinization of the suprasubduction-zone mantle along the Izu-Bonin-Mariana (IBM) convergent plate margin varies spatially, controlled largely by degree of faulting. Seismic velocity studies in both the Izu-Bonin and the Mariana portions of the system suggest that zones of serpentinized mantle are concentrated above the subducted slab and may have critical influence on the physical properties of the decollement. The metamorphic reactions occurring within the subduction zone are dependent on temperature, pressure conditions and on the composition of slab-derived fluids. Pore fluids recovered from active IBM forearc seeps show systematic changes in composition with distance from the trench, thus demonstrate a down-dip change in the nature of the reactions that release fluid-mobile elements from the subducting slab. These seeps also provide an extremophile microbiological environment, dominated by Archaea and host to unique Bacteria. Macrofauna communities have only been observed at the seeps farthest from the trench. These spatial characteristics of the IBM system can be linked to models of the reactions involved in the dewatering of the subducted slab and of the water-rock interactions in the overlying forearc lithosphere. These models are still controversial, to some degree, but the general serpentinization process is well understood. Serpentinization generates fluids with high pH, abiotic methane, and which react with seawater to precipitate chimney structures of dominantly carbonate and brucite. The same process and products are observed at the peridotite-hosted hydrothermal system at the south wall of the Atlantis Core Complex on the Mid-Atlantic Ridge (Lost City). Compositional differences between the Lost City site and the IBM seep sites reflect the subduction components that influence the latter. The temporal variability of the processes active at these sites has not yet been explored, however. The nature of fluids interacting with mid-ocean ridge peridotite probably has not changed substantially over the life-time of the Lost City site. By contrast, both the physical conditions and composition of fluids affecting the IBM forearc peridotite today have likely changed considerably from those occurring during the early stages of subduction. Two primary factors that must influence changes in the nature of metamorphic reactions within the suprasubduction-zone mantle with time as subduction proceeds are changes in temperature of the mantle region and changes in the flux rate and composition of slab-derived fluids.